1. Field of the Invention
The present invention relates to power-line-operated switch-mode or inverter-type power supplies with built-in means for preventing excessive ground-fault currents from flowing from the power supply output terminals.
More particularly, the invention relates to power-line-operated inverter-type fluorescent lamp ballasts of a type having built-in means for preventing excessive ground-fault currents from flowing from the ballast output terminals.
2. Description of Prior Art
Power-line-operated switch-mode or inverter-type power supplies are presently being used in a variety of applications. In particular, such power supplies are often used in connection with powering or ballasting fluorescent lamps and other gas discharge lamps.
An example of a fluorescent lamp ballast based on such a power-line-operated inverter-type power supply is provided in U.S. Pat. No. 4,277,726 entitled Solid-State Ballast For Rapid-Start Type Fluorescent Lamps and issued to Robert V. Burke.
When using such inverter-type ballasts in connection with the lamps most commonly used in fluorescent lighting fixtures, the voltages required at the sockets to start the fluorescent lamps are so high as potentially to constitute a substantial electric shock hazard to persons having to service such fixtures. To mitigate this hazard, whenever socket voltages exceed certain levels, protective measures have to be provided.
The essential shock hazard problem associated with a fluorescent lighting fixture relates to the situation where a person, who may be in contact with earth ground, is holding onto one end of a fluorescent lamp while the other end of the lamp is inserted into its socket.
Consequently, by using lamp sockets of a type that provides for disconnection of the socket voltages whenever a lamp is removed (so-called circuit-interrupting sockets), acceptably save operation will result. Alternatively, electrical isolation can be provided between earth ground and the ballast output to the lamp sockets.
In most applications of inverter-type ballasts, the resulting socket voltages are indeed high enough to require protective measures; and the only presently available commercially practicable solution (which is the solution provided for in said Burke Patent) is that of using an isolation transformer to provide electrical isolation between the power line input (which is connected with earth ground) and the ballast output to the lamp sockets. While this solution is indeed safety-wise acceptable, it involves substantial penalties in terms of ballast cost, size and weight, as well as in overall ballast efficiency.
Of course, circuit-interrupting sockets could be used; but that solution would require the use of non-standard and substantially more costly lamp sockets in addition to extensive added wiring within the fixture. Thus, the use of circuit-interrupting lamp sockets would constitute an even less attractive solution than would the use of an isolation transformer.
Another solution to the problem of electric shock hazard could involve the use of ground-fault circuit interruption means, which could be used for preventing excessive current from flowing from the ballast output terminals to earth ground.
However, at the present time, no electronic ballast with built-in means for preventing the flow of excessive ground-fault currents is presently available for purchase; nor has such a ballast been described in known published literature.